This invention relates to an oscillator.
Hitherto, there has been widely accepted an oscillator comprising an oscillation active element, particularly an oscillation transistor.
With that type of oscillator in which the emitter of an oscillation transistor is connected to the ground with respect to a D.C. signal, for example, a Hartley type oscillator, it is necessary to fix oscillation current or emitter current so as to stabilize an oscillating condition, that is, an oscillation signal frequency and oscillation signal output, even where changes occur, for example, in ambient temperature or power source voltage.
FIG. 1 indicates the prior art Hartley type oscillator using an NPN transistor as an active oscillation element. The collector of an oscillation transistor T.sub.r1 is connected to the ground through a capacitor C.sub.1 with respect to an alternating current and also to the plus side of a D.C. source E. The emitter of said oscillation transistor T.sub.r1 is connected to the junction of the inductive impedance elements Z.sub.1, Z.sub.2 of a tank circuit formed of said inductive impedance elements connected in parallel to a capacitive inductance element Z.sub.3. The base of said oscillation transistor T.sub.r1 is connected to the junction of the inductive impedance element Z.sub.1 and capacitive impedance element Z.sub.3 of the tank circuit through a capacitor C.sub.2 for feeding back power source current and also to one end of an emitter current-controlling resistor R.sub.1, the other end of which is connected to the junction of resistors R.sub.2, R.sub.3 constituting a potentiometer for dividing the voltage of the D.C. source E.
Where, with the above-mentioned arrangment, D.C. source current is supplied to the collector of the oscillation transistor T.sub.r1, and the base of the oscillation transistor T.sub.r1 is impressed with D.C. bias voltage represented by that portion of the voltage divided by the resistors R.sub.2, R.sub.3 which arises across both ends of the resistor R.sub.3, then an oscillation signal is issued from the output terminal OUT of the oscillator.
Obviously in this case, it is preferred that the oscillation signal be stabilized in frequency and power, and that even where changes occur in ambient temperature or power source voltage, oscillation current, or the emitter current I.sub.e1 of the oscillation transistor T.sub.r1 be kept constant.
The customary practice to stabilize the emitter current I.sub.e1 has been to provide a resistor on the collector or emitter side of the oscillation transistor T.sub.r1 to fix D.C. bias voltage impressed on the base thereof.
Where, however, a resistor is used on the collector side of the oscillation transistor T.sub.r1 a quantity of electricity charged in the tank circuit during one period of discharging of the capacitor C.sub.1 is decreased in proportion to a time constant defined by a product arrived at by multiplying the resistance of the resistor and the capacitance of the capacitor C.sub.1, undesirably resulting in a decline in an oscillation output. In contrast, where a resistor is provided on the emitter side of the oscillation transistor T.sub.r1, the selectivity Q of the tank circuit falls, also leading to a decrease in an oscillation signal output.